Modulation



Oct. 7, 1941.

I R. HOFER MODULATION Filed April 14, 1939 2 Sheets-Sheet 1 E DOUBLE/Z fiii 19- I Kl 5 CONTROL, I TRANSMITTER s 0 UR c5 0/: MODULA T/NG POTENTIALS INVENTOR RUDO FZ OFER BY he? WM.

ATTORNEY Oct. 7, 1941. R HOFER 2,257,799

MODULATION Filed April 14, 1939 2 Sheets-Sheet 2- CONTROL)TRANSMITTER 11 I V I l I Joy/ME /3 OF l.* i l l i ENERGY FREQUENCY CHANGER EU, HODULAT/A/G POTENTIALJ 57 INVENTOR ,QUDOLF HOFER BY%M ATTORNEY Patented Oct. 7, 1941 funken Gesellschaft fiir Drahtlose Telegraphic m. by H., Berlin, Germany, a: corporation of Germany ArrhoationApril .1 939, S l, o- 2671177- In Germany March 17,1938

This application. involves. a new. and improve modulation. methodforradio frequency trans. mitters.

Several methods have been disclosed in the earlier art which, are adapted to modulate a transmitter with. satisfactory efiiciency without the use of a lar e modulation transformer in the power stage, The method which is most widely known is predicated upon, the idea of employing a tube assembly which roughly operates at theboundary between the overs-voltage and undervoltage state for the supply of the carrier power. The positive modulation peaks are roduced by an additional or accessorytube assembly which is suitably coupled with the one first-.mentioned and which, in unmodulated condition, furnishes little or no power to the antenna, whereas, for the negative modulation peaks it stays blocked, while the first tube assembly is regulated down at the grid end. Another method. hasbeen dis-- closed which is, based upon a, similar principle, but which differs essentially from the foregoing in that the tubes, supplying the carrier power as far as feasible are adjusted .at an optimal efficiency, the modulation being eifected merely by a tube cascade which yields power to the antenna and another tube assembly which absorbs power from the antenna.

Inasmuch as underpractical operating conditions it is important that the tube furnishing the carrier power should operate under particularly favorable, conditions as. regards efliciency if, in view of the mostly prevailing rather low modulation percentages. satisfactory overall efficiency is to be secured, it, will, be understood that especially for this particular tube a saturation po.ten-. tial as low as, feasible in, comparison with the plate operating potential. is desirable. Where tubes with very large mutual. conductance are available, however, the carrier power, as well known, may be converted fromthe supplied power under particularly favorable, economic conditions, if the time of current supply, with extremely low residual plate potential, is as reduced as feasible compared with the period of the radio frequency. The time of current supply becomes especially small if a grid alternating potential amplitude as large as possible is used, and if in addition, the grid biasing voltage is made very high. But this, at all events, means a great increase in the control power.

An object of my invention is to provide means whereby the time during which radio-frequency impulses flow in the anode circuit of the carrier amplifier is short as compared to the, time duration ofihalf cycles of excitation voltageto therey ncrease the. effi ien y of operation. Another.

object of my invention is to accomplish the above and yet to use. lower grid biasing potentials and grid excitation voltages. A furtherobject of my invention, is, to provide an amplifier which, is excited by Voltages of a frequency which is a harmonic of the carrier and causes this amplifier to supply, power (plate modulate), to the anode of the carrier frequency. amplifier on peaks of the positive portion.v of the modulation cycle this energy being convertedv by the said carrier amplifier into energy of the fundamental frequency. In a, modification the amplifier supplies power to the carrier amplifier on the positive modulation.

- system arranged in accordance. with my invention, and Figures 2 and 3 are wiring diagrams of modulation systems arranged in accordance with,

my invention.

According tov the invention} derive from the. carrier wave to be modulated a voltage of basic or, fundamental frequency and ,a voltage of a frequency which, is a harmonic of said fundamental frequency. A combination voltage used to excite the grid of the modulated fundamental frequency amplifier is obtained from the basic it possible to obtain an object of the invention, i. e. the. desired narrow anode-current impulses,

in. the presence of a correspondingly lower grid biasing. voltage and also lower grid alternating potential peaks and thus lower control. power.

Use of a harmonic of the fundamental frequency,

of. course, is necessary to avoid current control at. another instant than the plate potential minimum. The fundamental frequency is represented by curve A of Figure 2 and the harmonic by r curve B. of Figure 2. Another object of the invention is, to insure modulation of the said fundamental frequency amplifier by an accessory amplifier or energy absorber which is not operated at. the fundamental frequency, but rather at a. higher harmonic thereof, say, the second byp feference. The accessory assembly operating with positive modulation does not furnish directly energy of useful (signal) frequency, but rather energy of harmonic frequency to the tube operating at carrier or fundamental frequency and this tube supplies this energy, with very high eificiency, at the fundamental frequency.

Supplying the modulation power to the carrier tube at a harmonic frequency is an advantage insofar as the efficiency of the whole equipment in the various modulation phases becomes somewhat more favorable than in the case of direct power delivery of fundamental frequency.

An exemplified embodiment of the scheme is schematically illustrated in Figure 2. Tube T has its grid 4 connected to transformer I coupled to wave energy source 3 and supplies the carrier power to the load R connected with the anode of tube T. The frequency of the wave energy supplied to grid 4 is the basic or fundamental frequency referred to herein. Tube T works approximately under limiting conditions, that is, at satisfactory eiliciency. The series mesh or network L2 C2 in the anode circuit is tuned to harmonic frequency, say, the second harmonic of the fundamental or basic frequency. When the grid 4 of tube II] is not modulated by potentials from transformer U1, tube R2 delivers hardly any power since it is suitably cut ofi or blocked by the potential supplied to the grid I I from source I. This tube is, however, furnished with a plate alternating potential of the second. harmonic frequency by way of the circuits L3, C3, L4, C2, connected to the anode 9 of tube R2. The grid alternating potential of tube T, in a Way hereinbefore described, is suitably composed of the fundamental wave supplied at In and the second harmonic voltage supplied by the transformer I9 from the doubler I5 which is coupled to the high frequency source 3. The amplitudes of these two frequencies are roughly alike, if desired (see curves A and B of Figure 1). Voltages of the second harmonic are also supplied to the grid circuit of R2 which has its grid I I coupled to a transformer I8 excited by current of double the fundamental frequency through phase shifter I6 and frequency doubler I5 coupled by transformer I3 to the source 3. The grid I I of tube R2 is also connected by transformer U2 to the modulation source. When the control or master power is comparatively low, very narrow plate current impulses are thus controlled, and this means a considerable rise of efiiciency, if tubes having a suitable mutual conductance are used. In the positive modulation phase, that is on the positive part of the modulation cycle, voltage is impressed substantially in phase by way of the transformers U1, U2 upon the grid circuits of tubes T and R2,

and tube R2 supplies energy of the second harmonic to the plate circuit of tube T, which, in turn, converts this applied second harmonic energy into wave energy of the fundamental frequency and supplies the same to the load R. During the negative modulation phase tube R2 is cut off or blocked, While tube T is modulated down to correspondingly low levels by a rise of the grid negative biasing voltage. The modulation may be effected also in an input stage with the result that the radio frequency grid potentials of tubes T and R2 fluctuate in amplitude at the rhythm of the modulation. In this connection, it is only necessary, for instance, to modulate the second harmonic, so that the grid voltage of tube R2 is modulated entirely, and that of tube T only in the second harmonic.

Another exemplified embodiment is schematically shown in Figure 3. Tube T again furnishes the carrier power to the load R under conditions of the highest efllciency obtainable, while tube R2, in the positive modulation phases, supplies power of the second harmonic to tube T in a way as before described, tube R3 in the negative modulation phases absorbing power from T. Inasmuch as its plate alternating potential becomes too low only for the highest modulation percentages to take current for energy consumption, it is necessary that only a rather low direct current voltage Us corresponding approximately to the saturation voltage of the tube, should be applied to the anode 25 of the said absorber tube R3. Modulation is effected again by Way of a transformer from the grid end or else from an input stage. Tube T need not necessarily be included in the modulation, though it may be given a small auxiliary modulation by way of the transformer U1, with the result that the efficiency becomes still better. Switch S is closed when modulation is not applied to the grid 4 of tube T. The phase of the grid alternating potential fed by transformer I8 to the grids II and 26 of the tubes R2 and R3 must be so that the plate alternating voltage of the second harmonic reaching the anode 5 of tube T has its maximum or minimum when the plate potential of the first harmonic presents its minimum. This condition will be realized when the grid alternating voltage U22 from the frequency changer F presents a 45- degree phase angle in relation to the grid alternating potential Ugt supplied to tube T.

To secure a straight modulation characteristic it is advantageous to open tube R2 at the grid end, in unmodulated state, to an extent so that, as the modulation phase becomes more and more positive, the modulation characteristic presents constant slope. In the presence of negative modulation phase, tube R3 sets in immediately and equalizes the curvature of the various modulation characteristics (Figure 4). In applicants arrangement it is noted that the need of large low frequency transformers for the power stage, say at L2 and L3, is obviated.

Having thus described my invention, what I claim is:

1. In a modulation system, a tube having input and output electrodes, means for impressing wave energy voltages to be modulated on said input electrodes, means for impressing modulating potentials on an electrode in said tube, means for impressing alternating voltages harmonioally related to said wave energy voltages on an electrode of said tube, and means for synchronizing the maximums of said alternating voltages with the maximums of said wave energy voltages.

2. A system as recited in claim 1 wherein said last electrode is the output electrode.

3. In a modulation system, a tube amplifier having input and output electrodes coupled in high frequency circuits including input and output circuits, means for causing high frequency wave energy to flow in said input circuit, means for varying the potential on an electrode in said tube in accordance with signals to thereby modulate said high frequency wave energy, means for applying a direct current operating potential to an electrode in said tube, and means for impressing on an electrode in said tube other high frequency Wave energy peaks of which occur simultaneously with peaks of the first high frequency wave energy on said last electrode to augment the same and thereby shorten the length of the resultant high frequency impulses on said last electrode without increasing the direct current operating potential or the amplitude of the high frequency wave energy caused to flow in said input circuit.

4. An arrangement as recited in claim 3 wherein said input electrodes include a control grid and cathode and wherein said electrode on which said' other high frequency wave energy is impressed is said control grid.

5. An arrangement as recited in claim 3 wherein said output electrodes include an anode and cathode and wherein said electrode on which said other high frequency wave energy is impressed is said anode.

pressed on said last electrode with peaks of the high frequency wave caused to flow in said input circuit.

7. The method of exciting an electrode of an electron discharge tube having a grid, an anode and a cathode to a given peak positive value with narrowed wave shape which comprises the steps of, impressing on said grid relative to said cathode a wave of fundamental frequency, impressing on said grid electrode relative to said cathode .a wave of a frequency which is a harmonic of said fundamental frequency, impressing on said anode relative to said cathode a wave of a frequency which is a harmonic of said fundamental frequency, and synchronizing said waves so that positive peaks of said waves occur at the same instant.

RUDOLF HOFER. 

